Semiconductor wafers used to fabricate integrated circuits are processed in vacuum chambers using common semiconductor processing techniques, such as CVD, sputtering and etching. The wafer must be securely held in a fixed position within the chamber, and means must be provided to carefully control the temperature of the wafer. In many cases, a cooling gas, such as helium is supplied to the backside of the wafer which serves as a heat transfer medium to assist in controlling wafer temperature.
Various techniques have been used in the past to hold the wafer in a desired position within the chamber. Early techniques involved mechanically clamping the topside of the wafer while providing cooling gasses to the backside; this approach is sometimes referred to as "top side clamping", but has not proved particularly effective, in large part because it suffers from the disadvantage of causing non-uniformity and particle inconsistencies at the extreme edge of the wafer.
More recently, electrostatic clamping has found increasing use. This technique makes use of the electrostatic attraction between objects of different electrical potentials, and commonly employs an electrostatic chuck as the device for providing electrostatic clamping forces. Electrostatic chucks are based on the forces of attraction between the charged plates of a capacitor. As in other types of wafer clamping and chucking, cooling gasses to control wafer temperature are supplied to the backside of the wafer, consequently the clamping force resulting from electrostatic forces must be sufficient to overcome the backside pressure on the wafer resulting from application of the heat transfer medium. The electrostatic chuck can be thought of as a capacitor with a conductive plate (a lower electrode fixed within the processing chamber), an insulating layer (a coating on the electrode), and another conductive plate (the wafer). In effect, the chuck is a parallel plate capacitor with a dielectric spacer. In its simplest form, this type of electrostatic chuck is referred to as a unipolar or monopolar electrostatic chuck. A unipolar electrostatic chuck is simply a conducting electrode with the wafer being employed as one of the conducting plates. In order to establish a complete circuit, an electrical connection to the wafer is required.
Currently, most unipolar electrostatic chucks use the gas plasma within the chamber as a conductor which completes the electrical circuit necessary to provide the electrostatic clamping force. A significant drawback of unipolar electrostatic chucks is the fact that the actual clamping force is not applied to the wafer until the wafer has been charged and the plasma has been generated in the chamber, the combination of which results in "chucking" the wafer on the electrode. Although an electrical conductor could be used to contact the wafer in lieu of the electrically conductive plasma, such contact could have serious consequences in terms of process reliability and reproducibility. After the wafer is electrostatically "chucked", the desired process is carried out in the chamber, following which it is necessary to unclamp the wafer. Wafer unclamping is achieved by removing the plasma from the chamber and bleeding off the electrical charge existing in the wafer; this step is commonly referred to as "de-chucking" the wafer. The speed and effectiveness of de-chucking are highly dependent upon the particular processes that have been previously carried out in the chamber. Simply turning off the voltage source to the chuck electrode with the plasma still present will not always result in instantaneous de-chucking since some residual charge may remain in the wafer. In any event, in addition to less than optimal process repeatability, the time required for chucking and de-chucking the wafer necessarily increases the overall time required to process a particular wafer, and thus reduces wafer throughput to the system.
Another type of electrostatic chuck referred to as a bipolar chuck is sometimes used, which is constructed from a pair of electrodes separated from the wafer by an insulating layer. The bipolar configuration has the disadvantage of being considerably more complex in construction than the unipolar configuration and typically generates a clamping force which is considerably less that of a comparably sized unipolar chuck.
It would therefore be desirable to provide a unipolar type electrostatic chuck which is capable of chucking and de-chucking the wafer without the need for using the plasma within the chamber to complete the circuit necessary for creating the electrostatic chucking force. The present invention is directed to satisfying this requirement.